1. Field of the Invention
The present invention relates generally to rotating magnetic disk drives and, more particularly, to a method of calibrating a position error signal (PES) for accurately moving an MR head in partial track increments from a position at which the PES equals zero and where the PES varies as a nonlinear function of real displacement due to a non-linear magnetic response characteristic of the MR head.
2. Description of the Related Art
A conventional disk drive has a head disk assembly ("HDA") including at least one magnetic disk ("disk"), a spindle motor for rapidly rotating the disk, and a head stack assembly ("HSA") that includes a head gimbal assembly (HGA) with a transducer head for reading and writing data. The HSA forms part of a servo control system that positions the transducer head over a particular track on the disk to read or write information from that track.
The industry presently prefers a "rotary" or "swing-type" actuator assembly which conventionally comprises an actuator body that rotates on a pivot assembly between limited positions, a coil that extends from one side of the actuator body to interact with a pair of permanent magnets to form a voice coil motor, and an actuator arm that extends from the opposite side of the actuator body to support the HGA.
Each surface of each disk conventionally contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo information is provided on this disk or another disk to determine the position of the head. The most popular form of servo is called "sampled servo" or "embedded servo" wherein the servo information is written in a plurality of circumferentially successive servo sectors that are angularly spaced from one another and interspersed between data sectors around the track. Each servo sector generally comprises a track identification (ID) field and a group of servo bursts (an alternating pattern of magnetic transitions) which the servo control system samples to align the transducer head with or relative to a particular data track. The servo control system moves the transducer toward a desired track during a coarse "seek" mode using the track ID field as a control input. Once the transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the transducer head over that track in a fine "track follow" mode. The transducer generally reads the servo bursts to produce a position error signal (PES) that is 0 when the transducer is at a particular radial position. The position where the PES=0 may or may not be at the data track center, however, depending on the magnetic characteristics of the transducer, the arrangement of the servo bursts, and the formula used to calculate the PES.
For many years, the industry has used single-gap inductive thin film heads where the same transducer or gap is used for reading and writing. More recently, however, the industry has begun using a Magneto-Resistive (MR) head which requires two separate transducers--an inductive transducer for writing and a Magneto-Resistive transducer for reading. An MR head offers an advantage over an inductive head in the recovery of data in disk drives requiring high areal density. However an MR head also presents a number of disadvantages. In particular, the separate read and write transducers are necessarily spaced apart from one another along the length of the supporting structure known as a "slider." As a result, their radial separation varies as the head is moved over the disk by a swing-type actuator.
This phenomenon generally requires distinct track following procedures for reading and writing. For example, during writing the read transducer track follows at the "null" position where the PES=0 and the write transducer records the data track offset by the amount of radial separation between the read and write transducers at this cylinder. For reading, therefore, the read transducer is "micro-jogged" away from the null position in order to align the read transducer with the data track, i.e. it track follows off-null at a position where the PES.noteq.0.
The jogging process can be troublesome, however, because the PES produced by the Magneto-Resistive transducer does not vary in linear proportion to the displacement from the position where the PES=0. This non-linear signal-to-displacement characteristic is a well known problem, but the solutions to date have a number of drawbacks as discussed below.
One known approach to calibrating the PES signal relative to displacement relies on a device called a servo track writer (STW). The STW is a manufacturing fixture used to write servo track information on the surfaces of disks in an HDA. The STW mechanically moves the actuator to a given reference position precisely measured by a laser interferometer. The HDA is then driven to write the servo track information for that position. The servo track writing process of precisely measured displacement and servo track writing is repeated to write all required servo tracks across the disk. During this process, the precise measurement of actual position can be used to characterize the PES read by the HDA's servo controller. This makes it relatively easy to measure the system gain for each head as a function of the actual displacement x and thereby generate a calibration factor. Unfortunately however, a STW is a very expensive piece of machinery, costing $100,000 or more, and using this process for calibrating system gain increases the time spent by the HDA in a STW which adversely impacts production time and cost. Finally, it is undesirable to calibrate the PES using a servo track writer because the calibration must be performed before the detailed self calibration process which the disk drive performs later in the manufacturing cycle. This is a significant disadvantage because the parameters of the servo channel may change due to adjustments in DC bias current applied to the MR transducer or other factors. Accordingly, the calibrations made with the servo track writer may become inaccurate or entirely invalid.
Another approach known to this inventor involves using the servo track writer to record special "calibration tracks" on each surface of each disk wherein a plurality of burst pairs define null points that are radially shifted from one another by precise, fractional track amounts that collectively provide accurate information about real displacement. Such calibration tracks beneficially allows for calibrating the PES signal after the drive is removed from the servo track writer. Unfortunately, however, the special calibration tracks take additional time on the servo track writer and occupy valuable space on the disk that could be used for data.
Accordingly, there is a need for a method of calibrating the response of the MR head which does not require a servo track writer and which may be performed when the disk drive requires detailed calibration without requiring special calibration tracks.